TypeDirectedLookup.scala

// The main idea with type-directed lookup is that argument values are
// looked up in a scope formed by appending the members in the companion
// object of the declared param type to the normal lexical scope.
//
// It is very similar to how implicit declarations in a companion-object
// are automatically addded to the implicit scope.
//
// Start reading from the `main` function. It's probably easier to follow
// the code.
object TypeDirectedLookup {
  trait CssValue {
    val value: String
  }

  case class CssColor(value: String) extends CssValue

  case class CssProperty(name: String, value: String) {
    override def toString = s"$name: $value"
  }

  case class CssName[V <: CssValue](name: String) {
    def :=(cssValue: V): CssProperty = CssProperty(name, cssValue.value)
  }

  // Make some factory functions available to type-directed lookup.
  object CssProperty {
    val color = CssName[CssColor]("color")
    val backgroundColor = CssName[CssColor]("background-color")
  }

  // Make some factory functions available to type-directed lookup.
  object CssColor {
    def rgb(r: Int, g: Int, b: Int): CssColor = CssColor(s"rgb($r, $g, $b)")
    def hex(h: String): CssColor = ???
    def hex(h: Int): CssColor = ???
  }

  // When this method will be used, values in argument position may be
  // resolved as members of the CssProperty's companion object.
  def div(properties: CssProperty*): String = {
    val declarations = properties.mkString(";\n  ")
    s"""div {\n  $declarations;\n}"""
  }

  def main(args: Array[String]): Unit = {
    // #1
    div(CssProperty.backgroundColor := CssColor.rgb(0,0,0))

    // #2
    {
      import CssColor._
      import CssProperty._
      div(backgroundColor := rgb(0,0,0))
    }

    // #3 type-directed lookup; won't compile because `color` and
    // `backgroundColor` are not in scope.
    div(
      color := rgb(0,0,0),
      backgroundColor := rgb(0,0,0)
    )

    // The previous call is conceptually similar to this desugaring.
    val css = div(
      ({ import CssProperty._; color }) := ({ import CssColor._; rgb(0,255,0)}),
      ({ import CssProperty._; backgroundColor }) := ({ import CssColor._; rgb(0,0,0)})
    )

    println(css)
  }
}